Fe--B--R permanent magnets containing B and Fe as their main components and no high-cost Sm or Co which are obtained by using light rare earth elements such as Nd and Pr which are plentiful resources have already been proposed as new high-performance permanent magnets that greatly exceed the maximum performance of conventional rare earth cobalt magnets (Japanese Patent Laid-open No. S59-46008/1984 and Japanese Patent Laid-open No. S59-89401/1984.)
The magnet alloys noted above have a Curie temperature ranging generally from 300.degree. C. to 370.degree. C. By replacing some of the Fe with Co, however, an Fe--B--R permanent magnet is obtained having a higher Curie temperature (Japanese Patent Laid-open No. S59-64733/1984, Japanese Patent Laid-open No. S59-132104/1984).
Also proposed is a Co-containing Fe--B--R permanent magnet that exhibits a Curie temperature that is at least as high as the Co-containing Fe--B--R permanent magnet noted above, and a higher (BH)max, wherein, in order to enhance the temperature characteristics, and especially to improve the iHc, at least one heavy rare earth element such as Dy or Tb is contained as the rare earth element (R) in some of the R in the Co-containing Fe--B--R permanent magnet wherein the R primarily consists of light rare earth elements as Nd and Pr, whereby, while maintaining an extremely high (BH)max of 25 MGOe or greater, iHc is raised higher (Japanese Patent Laid-open No. S60-34005/1985).
There are problems, however, in that the permanent magnets noted above, which are made from Fe--B--R magnetic anisotropic sintered bodies exhibiting outstanding magnetic properties, have a peculiar composition and structure, wherein the primary components are iron and rare earth elements that readily oxidize in air, wherefore, when they are built into magnetic circuits, due to oxides that are produced on the surface of the magnets, magnetic circuit output decline and variation between magnetic circuits are induced, and peripheral equipment is contaminated by the separation of the oxides from the magnet surfaces.
Thereupon, a permanent magnet has been proposed (in Japanese Patent Publication No. H3-74012/1991) wherein the surface of the magnet body is coated with an anticorrosive metal plating layer, by either an electrolytic or non-electrolytic plating method, in order to improve the anticorrosion performance of the Fe--B--R magnets noted above.
With these plating methods, however, the permanent magnet body is a porous sintered body, wherefore, in a pre-plating process, acidic solution or alkaline solution remains in the pores, giving rise to fears of degradation over time and corrosion, and the chemical resistance of the magnet body deteriorates, wherefore the magnet surface is corroded during plating so that adhesion and anticorrosion performance are impaired.
Even when the anticorrosive plating layer is provided, in anticorrosion tests in which samples are exposed to a temperature of 60.degree. C. and relative humidity of 90% for 100 hours, the magnetic characteristics proved to be very unstable, exhibiting 10% or greater degradation from the initial magnetic characteristics.
For this reason, it has been proposed (in Japanese Patent Publication No. H5-15043/1993) that, in order to improve the anticorrosion performance of Fe--B--R permanent magnets, an ion plating method, ion sputtering method, or vapor deposition method or the like be used to coat the surfaces of the magnets noted above with Al, Ti, or Al.sub.2 O.sub.3, and the anticorrosion performance thereby improved.
However, the Al.sub.2 O.sub.3 coating film has a coefficient of thermal expansion and ductility that differ from those of the Fe--B--R magnet bodies, wherefore adhesion is poor and, although the adhesion of the Al and Ti coatings is good, they are highly reactive, so that localized rusting occurs due to the external environment, and their anti-wear performance is also poor.
A method has also been proposed (Japanese Patent Publication No. H6-66173/1994) wherein, in order to improve the anticorrosion performance of the Al layer, the surface is subjected to a chromate treatment after the Al coating film, but the chromate treatment is problematic because it involves the use of 6-valence chromium which is environmentally toxic, and because treatment of the waste liquid is complex.